The (Al,Ga,In)N material system includes materials having the general formula AlxGayIn1-x-yN where 0≦x≦1 and 0≦y≦1. In this application, a member of the (Al,Ga,In)N material system that has non-zero mole fractions of aluminum, gallium and indium will be referred to as AlGaInN, a member that has a zero aluminum mole fraction but that has non-zero mole fractions of gallium and indium will be referred to as InGaN, a member that has a zero indium mole fraction but that has non-zero mole fractions of gallium and aluminum will be referred to as AlGaN, and so on. There is currently considerable interest in fabricating semiconductor light-emitting devices in the (Al,Ga,In)N material system since devices fabricated in this system can emit light in the blue-violet wavelength range of the spectrum (corresponding to wavelengths in the range of approximately 380-450 nm).
Semiconductor light-emitting devices fabricated in the (Al,Ga,In)N materials system are described, for example, by S. Nakamura et al in Jap. J. Appl. Phys., Vol. 35, pp. L74-L76 (1996). They are also described in U.S. Pat No. 5,777,350, which teaches use of the metal-organic chemical vapour deposition (MOCVD) growth technique to fabricate light-emitting devices in the (Al,Ga,In)N materials system. MOCVD (also known as metal-organic vapour phase epitaxy or MOVPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
Another known semiconductor growth technique is molecular beam epitaxy (MBE). In contrast to MOCVD, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the (Al,In,Ga)N system, an ultra-high vacuum (UHV) environment, typically around 1×10−3 Pa, is used. A nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and species providing aluminum, gallium and/or indium, and possibly also a suitable dopant species, are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The nitrogen precursor and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
At present, the majority of growth of high quality nitride semiconductor layers is carried out using the MOCVD process. However, MBE has recently been shown also to produce high-quality nitride optoelectronic devices. For example, US patent application 2005/0163179A1 teaches the use of MBE to fabricate semiconductor light-emitting devices in the (Al,Ga,In)N system.
In many cases it is desirable for the structure of an optoelectronic or electronic semiconductor device to include a layer having a current aperture. Such a layer generally has a high electrical resistance except for a portion which has a low electrical resistance; the high resistance portion of the layer blocks the flow of electrical current, whereas the portion with a low electrical resistance passes electrical current and thereby forms a “current aperture”. A current aperture provides spatial confinement of current within the device; in the case of an optoelectronic device, the device can be designed such that light is generated in regions of the device corresponding to the current aperture.
FIG. 1 is a schematic view of a known structure for a vertical cavity semiconductor laser device. The structure has a substrate 1, a first stack 2 of mirror layers, an active region 3 for generation of light, and a second stack 4 of mirror layers. The mirror layers can, for example, be suitable semiconductor or dielectric layers. The first and second stacks 2,4 each form a Bragg mirror for light emitted by the active region 3. The upper part of the laser structure, including the second stack of mirror layers, has a width and thickness that are much less than the width and thickness of the substrate 1 and the first stack 2 of mirror layers, so that the structure has a “step”. An electrical contact 5 is placed on the upper surface of the step, and a second contact (not shown) is placed on the underside of the substrate 1.
In the laser of FIG. 1 the active region 3 is wider than the second stack of semiconductor layers. It is therefore desirable to confine the generation of light to the part of the active region that is under the second stack of semiconductor layers, since light that is generated in a part of the active region that is not under the second stack of semiconductor layers will not contribute to the optical output of the laser. The laser is therefore provided with a current blocking layer 6. The current blocking layer 6 is provided between the contact 5 and the active region 3, and has a portion 7 with a high electrical resistance and a portion 8 with a low electrical resistance. The portion 8 with a low electrical resistance defines a current aperture. The low resistance portion 8 is substantially the same size and shape as the cross-section of the upper stack 4 of semiconductor layers, and is aligned with the upper stack 4 of semiconductor layers. Current flow through the current blocking layer is confined to the portion 8 with a low electrical resistance so that current flow through the active region is, as shown by the arrows in FIG. 1, confined to the part of the active region aligned with the low resistance portion 8 of the current aperture layer 6. Thus, in operation, generation of light is substantially confined to the part of the active region that is under the upper stack 4 of semiconductor layers.
Current apertures are especially beneficial in nitride semiconductor vertical cavity devices, where it is difficult to grow electrically conducting semiconductor Bragg mirrors, and where electrical current injection through the Bragg mirror is therefore not practical. The use of current apertures is not however limited to vertical cavity devices, and their usefulness in other nitride optoelectronic devices such as edge-emitting lasers or electronic devices such as transistors will be appreciated by anyone skilled in the art.
While methods exist to create current apertures in other III-V semiconductor material systems, for example by wet oxidation of AlAs layers in GaAs-based devices, existing methods of creating current apertures in devices in III-Nitride material systems are unsatisfactory.
Y. Gao et al. report, in Electr. Lett., Vol. 39, pp 148-149 (2004), the use of photoelectrochemical (PEC) wet etching of AlGaN/GaN layers to create air gap current apertures in electron transistors. While this procedure can in principle be applied in the manufacture of optoelectronic devices such as semiconductor laser diodes, this would require etching material close to the optical mode of the laser diode. It is expected that this would reduce the reliability and lifetime of the laser diode.
J. Dorsaz et al. propose, in Appl. Phys. Lett, Vol. 87, 072102, (2005), anodic oxidation of AlInN in III-Nitride devices to form the high-resistance regions of the current aperture layer. Using oxidised layers in nitrides has so far been reported to be unsuccessful in nitride semiconductor layer structures, and it is not known at present how well this proposed process would work. In particular, the reliability and lifetime of devices with oxidised layers may be affected.
U.S. Pat. No. 6,258,614 discloses a method of creating current apertures that uses selective annealing of p-type layers by lasers of different wavelength. This method requires p-type layers of different bandgaps, which initially are all highly resistive. A current aperture can be created by stacking two p-layers which have different bandgaps to one another and by bandgap-selective laser annealing. This method restricts the freedom in designing a nitride optoelectronic device, in that layers of appropriate bandgap, and matching lasers, have to be used. Annealing processes which are not bandgap-selective, such as electron beam annealing, cannot be used with this method. A further disadvantage is that, since all the p-layers are initially highly resistive, all the layers have to be annealed in the case of, for example, a LD cladding layer of 500 nm typical thickness. This may cause unintentional heating and degradation of the LD active region owing to the cladding layer thickness and its proximity to the active region.
B. Theys reports, in Defect and Diffusion Forum, Vols. 157-159, pp. 191-210, (1998), the hydrogenation of III-V semiconductors to create photonic devices. The paper discusses post-growth hydrogenation of dopants in surface layers, using hydrogen diffusion from a plasma source or hydrogen implantation, to create current confinement zones in the surface layer. In the vast majority of electronic or optoelectronic devices, however, the current aperture is not at a surface of the device but is disposed within the device.
U.S. Pat. No. 4,610,731 discloses a method of creating a low resistance region in each of a sequence of superposed AlGaAs/GaAs layers. Atomic hydrogen is used to neutralise shallow donors in the layers and create highly resistive layers, and a heat annealing process is then used to make a portion of the resistive layer n-type conducting. This method cannot, however provide a current aperture buried within the structure of a device, since every layer in the sequence of layers is heated during the annealing process.
The paper by Theys and U.S. Pat. No. 4,610,731 use hydrogen to vary the free-carrier concentration in a doped semiconductor layer. There are other reports of this technique.
For example, M. A. L. Johnson et al. report, in Mat. Res. Soc. Symp. Proc. Vol. 449, p. 215-220 (1997), on the MBE growth of GaN that is p-doped with magnesium while adding atomic hydrogen during the growth process. The material was studied using photoluminescence experiments, and evidence of p-dopant compensation in the material grown with added hydrogen was found.
M. S. Brandt et al. report, in Appl. Phys. Lett., Vol. 64, pp. 2264-2266 (1994), that the carrier concentration in an MBE-grown p-type GaN layer can be reduced by an order of magnitude using a post-growth hydrogen plasma process.
H. Amano et al. report, in Jap. J. Appl. Phys., Vol. 28, pp. L2112-L2114 (1989), the use of low-energy electron beam irradiation to lower the resistivity of MOCVD-grown Mg-doped GaN. As is now known, Mg-doped GaN grown by MOCVD is highly resistive owing to unintentional incorporation of hydrogen during MOCVD-growth.
U.S. Pat. No. 6,242,761 describes a method of fabricating a nitride semiconductor light-emitting device, in which a layer structure grown by MOCVD is disposed over part of the upper surface of an n-GaN contact layer. A first electrode is disposed over another part of the upper surface of the n-GaN contact layer. The layer structure contains an n-AlGaN cladding layer, an n-GaN guide layer, an active layer, a p-GaN guide layer, a p-AlGaN cladding layer, and a p-GaN contact layer. A second electrode is disposed over the p-GaN contact layer. In the method of U.S. Pat. No. 6,242,761, a reverse bias voltage is applied between the first and second electrodes to cause current to flow through the layer structure, so that Mg (the p-dopant) is activated in the portions of the p-type layers through which current flows; application of the voltage also causes hydrogen in the n-type layers to separate from the crystal in the portions of the n-type layers through which current flows. Low resistance regions are thus formed in the layers (corresponding to the current path through the layer structure), while the portions of the layers through which current does not flow remain with a high resistance.